The present invention relates to a process for regenerating iron-based hydrogen sulfide sorbents comprising treating a spent iron-based hydrogen sulfide sorbent with steam. In a preferred embodiment, the iron-based sorbent is first contacted with steam, preferably mixed with at least one of a hydrogen gas and an inert gas, such as nitrogen, followed by contacting with hydrogen under regeneration conditions.
The removal of sulfur from feedstocks is a fundamental process of the refining and petrochemical industries. One process for removing sulfur from a feedstock is hydrodesulfurization. Hydrodesulfurization involves the reaction of sulfur in the feedstock with hydrogen over supported noble metals, such as Pt, Pd, or supported non-noble metal catalysts, especially Co/Mo and Ni/Mo catalysts, at fairly severe temperatures and pressures thereby forming hydrogen sulfide.
The performance of the hydrodesulfurization catalysts, especially those containing a noble metal, can be inhibited by the presence of hydrogen sulfide. The use of sorbents to remove hydrogen sulfide produced during desulfurization improves the effectiveness of the overall hydrodesulfurization process.
The performance of a hydrogen sulfide sorbent depends on a variety of properties. Thermodynamics and kinetics of sulfidation clearly are important, because they determine the overall sulfur capacity before breakthrough at some predetermined level of hydrogen sulfide. Other important sorbent properties include stability and regenerability in extended use, the operating conditions required for regeneration, and the composition of the regeneration off-gas, which largely determines the choice of a downstream sulfur recovery process. A practical limitation on the use of any hydrogen sulfide sorbent is the ability to regenerate the sorbent. Zinc oxide, one of the most promising and widely studied sorbents, has a very high equilibrium constant for sulfidation, but it is difficult to regenerate. The use of zinc oxide may, therefore, be limited by economic constraints relating to the level of sulfur being processed, the reactor volumetrics required, and issues pertaining to removal and disposal of the spent sorbent. These limitations are relieved if the sorbent is capable of multicycle operation made possible by a means for regenerating the sorbent.
Regenerable solid sorbents currently used for treating hot gaseous streams are typically based on metal oxides and are regenerated under oxidizing conditions at temperatures frequently greater than about 600xc2x0 C. The regeneration of these sorbents using an oxidizing atmosphere requires an initial displacement of combustible organics, which not only is costly, but can also be hazardous.
Regeneration using hydrogen gas has been proposed as an alternative to oxidizing conditions for sorbents containing one or more of iron, cobalt, nickel, and/or copper. The use of hydrogen gas is effective for cobalt, nickel, and copper containing sorbents, but it is difficult to achieve substantially complete regeneration of an iron-containing sorbent using hydrogen alone. Therefore, methods are needed for the substantially complete regeneration of iron-containing sorbents using a non-oxidizing atmosphere.
The present invention provides a process for regenerating a spent iron-based hydrogen sulfide sorbent, comprising: exposing the spent iron-based hydrogen sulfide sorbent to a sufficient concentration of steam under conditions effective for the steam to regenerate the spent iron-based hydrogen sulfide sorbent.
In a preferred embodiment the spent iron-based sorbent is treated with steam in a first step, followed by being treated with hydrogen in a second step.
In another preferred embodiment of the present invention a mixture of steam and hydrogen are used in the first treatment step.
In still another preferred embodiment of the present invention a mixture of steam and hydrogen and an inert gas are used in the first treatment step.
The present invention uses steam, preferably in combination with at least one of hydrogen and an inert gas, to regenerate an iron-based spent hydrogen sulfide sorbent. In a more preferred embodiment, the spent iron-based sorbent is contacted with a mixture of steam and at least one of hydrogen and nitrogen gas in a first treatment step, followed by being treated with hydrogen in a second treatment step, both steps being conducted under sorbent regeneration conditions.
It is within the scope of this invention that the iron-based sorbents be either bulk iron materials, or iron on a suitable support, such as an inorganic oxide. Non-limiting examples of suitable iron-based supported regenerable sorbents include, but are not necessarily limited to: 5 Fe/Al2O3, 10 Fe/SiO2, and 20 Fe/ZrO2, wherein the numbers 5, 10, and 20 refer to the wt. % Fe based on the total weight of the sorbent. As previously mentioned, the hydrogen sulfide sorbent may be employed as a metal oxide or as bulk iron. If bulk iron is used as the sorbent in may be used as one or more type of finely divided skeleton metal, including Raney metals, ponderous metals, Rieke metals, and metal sponges.
If a support material is used, it is preferably one that will increase at least one of the surface area, pore volume, and pore diameter of the overall sorbent. Suitable support materials include, but are not limited to alumina, silica, zirconia, carbon, silicon carbide, kieselguhr, amorphous and crystalline silica-aluminas, silica-magnesias, aluminophosphates, boria, titania, and combinations thereof. Preferred support materials include alumina, silica, and zirconia. The iron or iron oxide may be loaded onto these support materials by conventional techniques known in the art. Such techniques include impregnation by incipient wetness, adsorption from an excess impregnating medium, and ion exchange. In a preferred embodiment, the regenerable sorbents are prepared by conventional impregnation techniques using aqueous solutions of iron halides, oxides, hydroxides, carbonates, nitrates, nitrites, sulfates, sulfites, carboxylates and the like. The iron or iron oxide loadings may vary with the quantity of sulfur to be adsorbed per cycle, the cycle frequency, and the regeneration process conditions and hardware. Iron loadings will range from about 2 wt. % to about 80 wt. %, preferably from about 3 wt. % to about 60 wt. %, and more preferably from about 5 wt. % to about 50 wt. %, based on the total weight of the regenerable sorbent. After impregnation onto a support, the sorbent typically is dried, calcined, and reduced; the latter may either be conducted ex situ or in situ, as preferred. The regenerable sorbent may comprise iron only, or iron with one or more additional metals.
In addition to its activity as a hydrogen sulfide sorbent, Fe is also a hydrocracking metal. Unless its hydrocracking activity is suppressed, Fe may cause hydrocracking of the other hydrocarbon stream being treated, leading to the production of low value light gas. The hydrocracking activity of the sorbent metal can be suppressed by incorporating from about 1 wt. % to about 10 wt. %, preferably from about 1.5 wt. % to about 7 wt. %, and most preferably from about 2 wt. % to about 6 wt. %, of a metal selected from Group IB or Group IVA of the Periodic Table of the Elements, such as Cu, Ag, Au, Sn, or Pb, preferably Cu.
Hydrogenolysis also can be suppressed by incorporating a small amount, preferably from about 0.01 wt. % to about 1 wt. %, of one or more of the elements selected from Group VIA of the Periodic Table of the Elements. The Periodic Table of the Elements referred to herein appears on the inside cover of the Merck Index, Twelfth Edition, Merck and Co., 1996.
Accordingly, the sorbent may be presulfided conventionally, for example, by exposing the virgin sorbent to dilute hydrogen sulfide in hydrogen at a temperature from about 200xc2x0 C. to about 400xc2x0 C. for about 15 minutes to about 15 hours, or until sulfur breakthrough is detected. Sulfur levels of the presulfided sorbent will range from about 0.01 to about 1.0 wt. %, preferably from about 0.02 to about 0.7 wt. %, most preferably from about 0.02 to about 0.5 wt. %. Alternately, the iron-based sorbent can be pre-sulfided by exposing the sorbent, preferably in its virgin, or fresh, form to a dilute aqueous solution of from about 1 vol. % to about 10 vol. % sulfuric acid under impregnation conditions.
Regeneration of the sorbent using a reducing environment generally requires more severe temperatures than those employed during the hydrodesulfurization (HDS) reaction. Typical regeneration temperatures are from about 100xc2x0 C. to about 700xc2x0 C., preferably from about 250xc2x0 C. to about 600xc2x0 C., and more preferably from about 275xc2x0 C. to about 550xc2x0 C. A most preferred regeneration temperature range is from about 500xc2x0 C. to about 550xc2x0 C. The regeneration process is operable over a range of temperatures and pressures consistent with the intended objectives in terms of product quality improvement and consistent with any downstream process with which this invention is combined in either a common or sequential reactor assembly. Operating pressures may range from about 0 to about 3000 psia, preferably from about 50 to about 1000 psia, at H2 gas rates of from about 10 to about 2,000 SCF/hr/lb. of sorbent (standard cubic feet per hour per pound), preferably about 100 to about 1,000 SCF/hr/lb of sorbent.
Hydrogen is a preferred component of the regeneration process of the present invention. Hydrogen may be supplied from a hydrogen-containing stream of pure hydrogen or inert gases, preferably nitrogen. It is preferred that the hydrogen-containing stream be substantially free of sulfur, which can be achieved by conventional technologies known in the art. The hydrogen-containing stream will contain from about 50 vol. % to about 100 vol. % hydrogen, preferably from about 70 vol. % to about 100 vol. % hydrogen, and more preferably from about 80 vol. % to about 100 vol. % hydrogen, with any remainder being an inert gas or saturated light hydrocarbon gases.
When hydrogen gas only is employed as the regeneration medium, the iron may be incompletely regenerated. However, substantially complete regeneration of an iron-based sorbent material, such as Fe/ZrO2, may be achieved for at least several cycles by using a combination of steam and hydrogen. While regeneration may be performed under conventional conditions, a preferred process for regenerating a spent iron-based sorbent in accordance with the present invention is to heat the spent iron-based sorbent from about 400xc2x0 C. to about 600xc2x0 C., preferably to about 500xc2x0 C., in a mixture comprising from about 5 vol. % to about 95 vol. % steam, preferably from about 10 vol. % to about 80 vol. % steam, more preferably from about 15 vol. % to about 60 vol. % steam, most preferably from about 20 vol. % to about 50 vol. % steam. In another preferred embodiment, a mixture comprising about 20 vol. % steam combined with hydrogen and optionally one or more inert or light hydrocarbon gases is heated to about 500xc2x0 C. for about 0.25 hour to about 10 hour, preferably for about 1.5 hour. Thereafter, the iron-based sorbent is exposed to hydrogen at regeneration conditions for about 30 minutes.
Among the properties desired in a regenerable hydrogen sulfide sorbent are capacity to absorb hydrogen sulfide, regenerability, and the retention of both qualities over multicycle adsorption-regeneration sequences. Although it is preferred that both capacity and regenerability for a given sorbent approach about 100%, it should be understood that this level is not a requirement for a commercially effective regenerable hydrogen sulfide sorbent. A capacity and regenerability that allow a frequency of regeneration that is reasonable and compatible with the overall process objective are acceptable and adequate. With this qualification in mind, an xe2x80x9ceffective regenerated capacityxe2x80x9d is from about 5% to about 100%, by weight, of a first cycle capacity, preferably from about 10% to about 100% of a first cycle capacity, most preferably from about 20% to about 100% of a first cycle capacity. A xe2x80x9cfirst cycle capacityxe2x80x9d refers to the sorbent capacity of a fresh or xe2x80x9cvirginxe2x80x9d sorbent material.
In a preferred embodiment, the iron-based sorbent is used in distillate and naphtha hydrodesulfurization (HDS) processes to sorb hydrogen sulfide as it is generated. Typical hydrodesulfurization processes are well known in the art and are represented by the disclosures of one or more of U.S. Pat. Nos. 5,925,239, 5,928,498, and/or 5,935,420, all incorporated herein by reference.
Typical hydrodesulfurization conditions include temperatures from about 40xc2x0 C. to about 500xc2x0 C. (104-930xc2x0 F.), preferably about 200xc2x0 C. to about 450xc2x0 C. (390-840xc2x0 F.), and more preferably about 225xc2x0 C. to about 400xc2x0 C. (437-750xc2x0 F.). Operating pressures include about 50 to about 3000 psig, preferably about 50 to about 1200 psig, and more preferably about 100 to about 800 psig at gas rates of about 50 to about 10,000 SCF/B, preferably about 100 to about 750 SCF/B, and more preferably about 500 to about 5000 SCF/B. The liquid hourly space velocity may be varied over the range of about 0.1 to about 100 V/V/Hr, preferably about 0.3 to about 40 V/V/Hr, and more preferably about 0.5 to about 30 V/V/Hr. The liquid hourly space velocity is based on the volume of feed per volume of catalyst per hour, i.e., V/V/Hr.
Various sorbent bed configurations may be used in the practice of the present invention. Examples of suitable bed configurations include, but are not necessarily limited to, fixed beds, non-fluidized moving beds, fluidized beds, or a slurry of HDS catalyst and sorbent in a continually stirred tank reactor (xe2x80x9cCTSRxe2x80x9d) or slurry bubble column.
Fixed bed configurations may be operated in either of cocurrent and countercurrent modes, i.e., with hydrogen-containing treat gas flowing over the HDS catalyst in the same or opposite direction to the sulfur-containing feed. In another embodiment, the hydrogen-containing treat gas is employed in a xe2x80x9conce-throughxe2x80x9d arrangement and is, therefore, not recycled. Countercurrent HDS arrangements may be preferred in cases where increased contacting between the sulfur-containing feed, treat gas, and catalyst would be desired and in cases where H2S stripping would be beneficial. Fluidized beds may be advantageous in conjunction with processes where continuous regeneration of the sorbent is needed. In addition, flow-through, fluidized bed technology that includes a disengaging zone for catalyst and sorbent may be useful to regenerate sorbent particles.
Those skilled in the art are aware that the choice of bed configuration for an HDS catalyst and a sorbent depends upon the objective of the overall process, particularly when the process is integrated with one or more subsequent processes, or when the objective of the overall process is to favor the selectivity of one aspect of product quality relative to another. However, it should be noted that it is preferred that the sorbent not be placed upstream of the HDS catalyst.
A preferred embodiment uses a stacked bed configuration with a swing reactor designed to permit regeneration of spent sorbent while a fresh sorbent is placed in service. In a stacked bed configuration, the HDS catalyst is stacked, or layered, above and upstream of a sorbent bed. The stacked beds either may occupy a common reactor, or the HDS catalyst may occupy a separate reactor upstream of the reactor containing the sorbent. Such a dedicated reactor sequence would be useful, for example, when it is desired that the HDS catalyst and the sorbent be exposed to different reactor temperatures.
In another embodiment, the sorbent and HDS catalyst are used in a mixed bed configuration. In this configuration, particles of the HDS catalyst are intimately intermixed with those of the sorbent. In both the stacked bed and the mixed bed configurations, the HDS catalyst particles and the sorbent particles may be of similar or identical shapes and sizes. The particles of one component may also differ, for example, in shape, density, and size from the particles of the second component. The use of particles having different sizes may be employed, for example, when a simple physical separation of the bed components is desired upon discharge or reworking. It should be noted that when the sorbent and the HDS catalyst are used in combination, that undesirable desulfiding of the catalyst may result from sorbent regeneration. In such cases, it may be desirable to, for example, adjust the regeneration conditions to lessen the affects of catalyst desulfiding to provide for re-sulfiding the catalyst prior to re-use, to separate the catalyst and sorbent prior to sorbent regeneration, or some combination thereof. Resulfiding may also be accomplished when the catalyst is exposed to the sulfur-containing feed.
In yet another embodiment, the two components are blended together to form a composite particle incorporating both the HDS catalyst and the iron-based sorbent. For example, a finely divided, powdered Pt on alumina catalyst is uniformly blended with a regenerable iron-based sorbent and the mixture is formed into a common catalyst particle by conventional techniques. Or, the regenerable sorbent may also be incorporated into the support, and Pt, for example, may be impregnated onto the sorbent containing support, such as alumina.
In another two component configuration, a support, preferably an alumina support, is impregnated with a noble metal, preferably a Pt salt and iron. Both metals may be distributed uniformly throughout the resulting catalyst particle, or the iron component may be deposited preferentially on the outside of the particle to produce a rim, or eggshell, sorbent- or HDS catalyst-rich zone.
A three-component bed configuration may also be used. In one embodiment, denoted as mixed/stacked bed, a mixed bed containing the HDS catalyst and iron-based sorbent is configured upstream of a single bed of HDS catalyst. In another embodiment, known as a stacked/stacked/stacked configuration, the three components are layered from top to bottom as follows: HDS catalyst/iron-based sorbent/HDS catalyst. In one embodiment, three component systems may occupy a common reactor. In another embodiment, a three-component system may be used in a two-reactor train in which the HDS catalyst/sorbent occupy a lead reactor in a mixed or stacked configuration and a HDS catalyst occupies the tail reactor. This arrangement allows for the operation of two reactor sections at different process conditions, especially temperature, and imparts flexibility in controlling process parameters such as selectivity and product quality.
The composition of the bed is independent of configuration and may be varied in accordance with the specific or integrated process to which the invention is applied. If the capacity of the sorbent is limiting, the composition of the bed must be consistent with the expected lifetime, or cycle, of the process. These parameters are in turn sensitive to the sulfur content of the feed being processed and to the degree of desulfurization desired. For these reasons, the composition of the bed is flexible and variable, and the optimal bed composition for one application may not serve an alternative application equally well. In general, the weight ratio of the sorbent to the hydrodesulfurization catalyst may range from about 0.01 to about 1000, preferably from about 0.5 to about 40, and more preferably from about 0.7 to about 30. For three component configurations, these ranges apply to the mixed zone of the mixed/stacked arrangement and to the first two zones of the stacked/stacked/stacked design. The hydrodesulfurization catalyst present in the final zone of these two arrays is generally present at a weight ratio that is equal to or less than the combined weight compositions of the upstream zones.
The process of the present invention may be used as a stand-alone process for, for example, fuels, lubes, and chemicals applications. Alternately, the process may be combined and integrated with other processes in a manner so that the net process affords product and process advantages and improvements relative to the individual processes not combined. The following embodiments are included to illustrate, but not limit, uses for the process of this invention.
Processes relating to fuels processes include: desulfurization of gasoline range feed and product streams; desulfurization of distillate streams; desulfurization of FCC streams preceding recycle to 2nd stage process; desulfurization of hydrocracking feeds; multi-ring aromatic conversion through selective ring opening; aromatics saturation processes; sulfur removal from natural, synthesis, and recycle gas streams and from field condensate streams. Processes relating to the manufacture of lubricants include: hydrocracking, product quality improvement through mild finishing treatment; optimization of white oil processes by decreasing catalyst investment and/or extending service factor. Processes relating to chemical processing include: substitute for environmentally unfriendly nickel based hydroprocesses; preparation of high quality feedstocks for olefin manufacture through various cracking processes and for the production of oxygenates by oxyfunctionalization processes; production of solvent and polymer grade olefins and aromatics.